1. Field of the Invention
The present invention is directed to the encoding of a video data stream, particularly of a digitalized image.
2. Description of the Related Art
Greater and greater significance is being accorded to an efficient encoding of digitalized image data (video data stream) in the field of telecommunications technology, specifically in the area of image processing. The encoding of the data should be implemented such that an optimally great compression of the information is achieved given optimally little information loss.
Various methods for encoding a video data stream are known, for example MPEG (Moving Picture coding Experts Group), see the publication by D. Le. Gall, The Video Compression Standard for Multimedia Applications, Communications of the ACM; the publication by H. Sun, Architectures for MPEG Compressed Bitstream Scaling, IEEE Transactions on Circuits and Systems for Video Technology and the publication by A. Jain, Image Data Compression: A Review, Proceedings of the IEEE; JPEG (Joint Photographic Experts Group), see the publication by G. Wallace, The JPEG Still Picture Compression Standard, Communications of the ACM, the standard H. 261, see ITU-T H.261, International Telecommunication Union, standard H.263, see the publication by Ming Liou, Overview of the pxc3x9764 kbit/s Video Coding Standard, Communication of the ACM.
These methods, which are referred to as block-based image encoding methods, employed principles of prediction encoding and of transformation encoding and entropy encoding.
In the prediction, encoding method difference images are generated by subtraction of predicted image data from the original image data to be encoded.
What is referred to as a motion-compensated prediction is employed for the prediction. The fundamentals of the motion estimation required for this purpose and their application for motion-compensated prediction are familiar to a person skilled in the art, see the publication by A. N. Netravali und J. D. Robbins, Motion Compensated Television Coding: Part I, Bell System Technical Journal. The motion estimation ensues for an image block to be encoded such that the luminance information (brightness information) that is respectfully allocated to a picture element of the image of the image block to be encoded is compared to luminance information of an area having the same shape in a stored, chronologically preceding image. The comparison usually ensues by forming the absolute differences of the individual luminance values. The comparison ensues for the image block to be encoded to a plurality of areas of the preceding image, these being referred to below as preceding image blocks. The difference images now only contain the difference of the luminance values of the image block and the luminance values of the preceding image block that coincides xe2x80x9cbestxe2x80x9d in the motion estimation.
The topical correlations present in the difference images between neighboring picture elements are utilized with the assistance of a suitable transformation, for example with the assistance of discrete cosign transformation (DCT). The transformation encoding employed supplies transformation encoding coefficients that are subjected to a quantization and to an entropy encoding. Subsequently, the transformation encoding coefficients are transmitted to a receiver, whereby the entire encoding method is implemented in an inverse way. After implementing the decoding, step direct information about the picture elements are again available at the receiver.
Methods for what is referred to as object-based image encoding are known from the publication ISO/IEC JTC1/SC29/WG11, MPEG-4 Video Verification Model Verson 5.0 Doc. In these object-based methods. In these methods, methods for motion estimation and transformation encoding are likewise utilized.
All difference images are always subjected to a transformation encoding in the known methods for image encoding described above. When individual blocks are very similar as a result of the motion estimation, the difference of the image block to be encoded at the corresponding image block of the preceding image yields extremely small values that can be quantized into zero in the quantization of the transformation encoding coefficients under certain circumstances. Previously, however, the differences of the luminance values of the individual image blocks were subjected to a transformation encoding in the known, block-based encoding methods.
The present invention based on the problem of reducing the outlay for encoding and decoding a video data stream.
The problem is solved with the method for encoding a digitalized image with image segments that include picture elements to which a respective encoding information is allocated, upon employment of a stored, chronologically proceeding image having preceding image segments that have preceding picture elements to which a respectively preceding encoding information is allocated, whereby a preceding image segment is selected depending on a predetermined start vector that indicates a topical shift of the image segment relative to the preceding image segment; whereby an error criterion is formed between encoding information of the image segment and preceding encoding information of the preceding image segment; whereby the error criterion is formed such that the differences of the encoding information of different picture elements are differently weighted; whereby a check is carried out to see whether the error criterion is smaller than a first threshold; whereby, when the error criterion is smaller than the first threshold, the image segment is not subjected to any residual error encoding; whereby, otherwise, the image segment is subjected to the residual error encoding and to an entropy encoding.
The invention also provides an apparatus for encoding a digitized image with image segments that include picture elements to which an encoding information is respectively allocated, upon employment of a stored, chronologically preceding image with preceding image segments that have preceding picture elements to which respectively preceding encoding information is allocated, comprising a, processor unit that is configured such that a preceding image segment is selected proceeding from a predetermined start vector with which a topical shift of the image segment relative to the preceding image segment is indicated; an error criterion between encoding information of the image segment and preceding encoding information of the preceding image segment is formed; a check is carried out to see whether the error criterion is smaller than a first threshold; when the error criterion is smaller then the first threshold, the image segment is not subjected to any residual error encoding; otherwise, the image segment is subjected to the residual error encoding and an entropy encoding.
The method as described above for encoding a digitalized image with image segments that comprise picture elements to which respective encoding information is allocated ensues upon employment of a stored, chronologically preceding image with preceding image segments that comprise preceding picture elements to which a respective preceding encoding information is allocated. A preceding image segment is selected dependent on a start vector that indicates a topical shift of the image segment relative to the preceding image segment. An error criterion between the encoding information of the image segment and the preceding encoding information of the preceding image information is formed and a check is carried out to see whether the error criterion is lower than a first threshold. When the error criterion is lower than the first threshold (S1), the image segment is not subjected to any residual error encoding. Otherwise, the image segment is subjected to a residual error encoding and to an entropy encoding.
What is to be understood by a chronologically preceding image is the preceding, reconstructed image. What is to be understood by a preceding image segment and a preceding picture element are an image segment or, respectively, a picture element of the preceding image.
What is to be understood in the present document by a residual error encoding is an efficient encoding of the difference image information, for example a DCT based transformation encoding, a wavelet transformation encoding, a quad-tree encoding, a fractional encoding, a vector quantization, a difference pulse-code modulation (DPCM), etc.
What is to be understood by encoding information is brightness information (luminance information) or color information (chrominance information) allocated to a picture element.
The apparatus as described above comprises a processor unit that is configured such that the above-described method steps are implemented.
The apparatus can be a standard computer in which the above-described method is stored in the form of a computer program. However, it can also be realized as a specific hardware that is employed for image encoding.
Advantageous developments of the invention are provided by a method whereby the start vector is a zero vector, is a predicted vector that is determined with a prediction for the image segment, or a motion vector that is determined with a motion estimation for the image segment. When the error criterion is smaller then the first threshold, the following steps are implemented: at least one further error criterion is formed for the image segment between the image segment and the preceding image segment; when the second error criterion is smaller then a second threshold, the image segment is not subjected to any residual error encoding; otherwise, the image segment is subjected to the residual error encoding and to an entropy encoding.
In a preferred embodiment, the image segment is subjected to a quantization in the residual error encoding. At least one of the thresholds is adaptively fashioned Alternately, at least one of the thresholds is adaptively fashioned dependent on a quantization parameter. The invention provides that at least one of the following criteria is employed as further error criterion: the difference of the color information of the picture elements of the image segment and the color information of the preceding picture elements of the preceding image segment; the difference of the luminance information of the picture elements of the image segment and the luminance information of the preceding picture elements of the preceding image segment; a topical expanse of the difference of the encoding information of the picture elements of the image segment and the encoding information of the preceding picture elements of the preceding image segment.
The method is further defined whereby, for determining the topical expanse of the difference of the encoding information of the picture elements of the image segment and the encoding information of the preceding picture elements of the preceding image segment, a check is carried out at least once to see whether the difference of the encoding information given a plurality of picture elements as a first plurality is greater then a third threshold; the image segment is subjected to the residual error encoding and to the entropy encoding when the difference of the encoding information in more picture elements then the first plurality is greater then the third threshold.
For determining the topical expanse of the difference of the encoding information, a check is carried out in a plurality of steps to see whether the difference of the encoding information in more picture elements as a respectively further plurality is greater then a respectively further threshold, the image segment is subjected to the residual error encoding and the entropy encoding when the difference of the encoding information in more picture elements then a further plurality is greater then the further threshold. At least one of the pluralities and/or at least one of the thresholds is adaptively fashioned. At least one of the pluralities and/or the third threshold and/or further thresholds are adaptively fashioned dependent on a quantization parameter. The further error criterion is determined such that the differences of the encoding information of different picture elements are differently weighted. The weighting ensues such that differences of encoding information of picture elements that are located in an edge region of a prescribable size of an image segment are weighted higher then differences of encoding information of picture elements outside the edge region.
At least one of the following criteria can be employed as a further error criterion:
the difference of the color information of the picture elements of the image segment and the color information of the preceding picture elements of the preceding image segment;
the difference of the luminance information of the picture elements of the image segment and the luminance information of the preceding picture elements of the preceding image segments;
a topical expanse of the difference of the encoding information of the picture elements of the image segment and the encoding information of the preceding picture elements of the preceding image segment.
These criteria for forming the second error criterion correspond to different tests in view of different artifact types that arise due to the known image encoding methods.
By taking the color information or, respectively, luminance information into consideration in the comparison of the image segments, the comparison is expanded by a further criterion and, thus, the result of the comparison is improved.
For simplifying the method and, thus, for reducing the calculating capacity required for the implementation of the method, it is also advantageous in a development to check at least once for determining the topical expanse of the difference as to whether the difference of the encoding information given a plurality of picture elements as a first plurality is greater than a third threshold T1. The image segment is subjected to the residual error encoding and to the entropy encoding when the difference of the encoding information given a plurality of picture elements as the first plurality is higher than the third threshold.
A check can also be carried out in a plurality of steps as to whether the difference of the encoding information given a plurality of picture elements as a respectively further plurality is greater than a respectively further threshold.
It is advantageous for improving the image quality to fashion at least one of the pluralities and/or of the thresholds adaptively, preferably dependent on a quantization parameter. What is thus achieved is that, given a rougher quantization, a higher plurality of picture segments likewise need not be subjected to a motion estimation, a residual error encoding and an entropy encoding and, thus, further calculating capacity for the implementation of the method is reduced.
Another improvement of the results that are achieved is achieved in a development in that the error criterion is determined such that the differences of the encoding information of various picture elements are differently weighted.
Since artifacts often appear in an edge region of an image segment, it has proven advantageous to implement the weighting such that differences between picture elements that are located in an edge region of prescribable size in an image segment are weighted higher than differences of picture elements outside the edge region.
Even though the invention in the exemplary embodiments is described below on the basis of a block-based image encoding method, it can also be advantageously utilized without further ado in object-based image encoding methods.
What is understood below by an image segment is a quantity of picture elements of an arbitrary shape that are grouped together. In block-based image encoding methods, the image segments comprise a rectangular shape, for example a quadratic shape given the method according to the MPEG 2 standard, that respectively contain 8xc3x978 picture elements (image block) or 16xc3x9716 picture elements (macro-block). In block-based image encoding methods, the image segments are referred to as image blocks.